the netchan of darkplaces is the weakest part of the dp7 protocol - packets are still sent 100% compatibily with nq including separate packets for reliable and unreliable data, and the demo format itself is completely unchanged.
you can easily concatinate the extra data on the end without issues.
there's really no need to care about any of the svcs other than the new one that you wish to add.
tweak the code given in mandel's reply here:
http://www.celephais.net/board/view_thread.php?id=60932 #9
note how the only svc it actually cares about is the disconnect one. which is always in a single packet
.